Method for obtaining organosilanes using a distribution reaction

ABSTRACT

A redistribution reaction between organohydrochlorosilanes and optionally chlorinated organosilanes is run in the presence of a catalyst in the solid state which comprises an alumina with an alkali metal or alkaline earth metal content expressed in ppm of the corresponding oxides of less than or equal to 500 ppm.

The subject of the present invention is an improved process forobtaining organosilanes and it relates in particular to an improvedprocess for obtaining organosilanes in which a so-called redistributionreaction is involved. More particularly, the present invention relatesto an improved process for obtaining organosilanes, involving aredistribution reaction between a chlorinated organohydrosilane and anorgano-substituted and optionally chlorinated silane to give a productcomprising a redistributed chlorinated organohydrosilane which isextracted from the reaction medium by distillation.

Without limitation, the present invention is aimed quite especially at aredistribution reaction between an alkylhydrodichlorosilane and atrialkylchlorosilane to give a product comprising a redistributeddialkylhydrochlorosilane. This redistributed dialkylhydrochlorosilane isa synthesis reagent which is highly prized in a great many and variedapplications, examples being the preparation of organosilicon monomersor more condensed base compounds.

The dialkylhydrochlorosilane is one of the by-products of the synthesisof alkylchlorosilanes in accordance with a conventional and well-knownprocedure which consists in reacting alkyl chloride with silicon in thepresence of a copper catalyst to form alkylchlorosilanes. In thissynthesis, the dialkyldichlorosilane is the main product. In addition tothe dialkylhydrochlorosilane by-product specified above, compounds ofthe trialkylchlorosilane, alkyltrichlorosilane andalkylhydrodichlorosilane type are also obtained.

In view of the industrial interest of these products in the chemistry ofsilicones and especially of dialkylhydrochlorosilanes such asdimethylhydrochlorosilane, numerous proposals have seen the light of dayfor methods of obtaining these by-products. One of the few proposalswhich has proved itself in this respect is that which consists incarrying out a redistribution reaction between, for example, analkylhydrodichlorosilane and a trialkylchlorosilane or between analkylhydrodichlorosilane and a tetraalkylsilane. This redistributionleads to the specified dialkylhydrochlorosilanes which are extractedfrom the reaction medium by distillation.

In this context, numerous redistribution reactions of organosilanes,which cut and redistribute the silicon-alkyl, silicon-chlorine and/orsilicon-hydrogen bonds, in the presence of various catalysts such asLewis acids, are known. French Patent FR-A-2 119 477 is a goodillustration of this technique for preparing dialkylhydrochlorosilanesby redistribution/distillation. In accordance with the teaching of thispatent, methylhydrodichlorosilane and trimethylchlorosilane are reactedin a molar ratio of methylhydrodichlorosilane to trimethylchlorosilaneof the order of 0.5 and in the presence of a catalyst consisting ofAlCl₃. The reaction mixture is placed in a reactor under an autogenouspressure of the order of 3 to 5×10⁵ Pa and is held for several hours ata temperature of the order of 85 to 170° C. The Applicant has repeatedthis prior art method and has observed that it involves a reaction inhomogeneous catalysis in which the reacting substances and the catalystform a single liquid phase. After the redistribution reaction has beenimplemented, the redistributed dimethylhydrochlorosilane is separatedfrom the reaction mixture by distillation and, at the end of thedistillation step, a distillation residue comprising the catalyst isleft. The said residue is generally in the form of a suspension, since agreater or lesser proportion of the catalyst (which will depend on theamount employed at the start) is in the solid state in the residue. Atthe end of the process, owing to the dual state of the catalyst (solidstate and dissolved state), the separation of AlCl₃ is made difficult,which considerably complicates the implementation of the process, and itis therefore quicker to destroy the catalyst by conducting an acidic orbasic hydrolysis of the distillation residue. It must, however, be bornein mind that results of this kind are not satisfactory from thestandpoint of industrial profitability: the aluminium chloride cannot berecycled, owing to its hydrolysis, and, furthermore, it presents theproblem of aqueous effluents which can be awkward.

In the light of this knowledge, one of the essential objectives of thepresent invention consists in the development of a novel process forobtaining organosilanes which involves a heterogeneously catalysedredistribution reaction between a chlorinated organohydrosilane and anorgano-substituted and optionally chlorinated silane. This novel processmust employ a catalyst which remains in the solid state in the presenceof the reacting substances and must lead, at the end of the reaction, toa reaction medium comprising a solid phase, the catalyst, which caneasily be recycled in its entirety into a new operation, and a liquidphase comprising a redistributed chlorinated organohydrosilane which isrecovered in conventional manner by distillation (for example).

Another essential objective of the invention is to provide a process ofthe type specified above which is particularly simple to implement andeconomic.

These various objectives are achieved by the implementation of theprocess according to the invention, which involves the use of aneffective amount of an alumina-based catalyst.

The document EP-A-0 743 315 proposed a process for redistributing amixture of methylsilanes by heterogeneous catalysis over alumina for thepurpose of increasing the dimethylhydrochlorosilane and/ortrimethylchlorosilane concentration of the said mixture. The startingmaterials referred to in this prior art relate only to mixtures ofmethylchlorosilanes (approximately 5 in number) and tetramethylsilanehaving low boiling points (methylsilanes boiling between 35 and 70° C.under atmospheric pressure are involved) as are separated bydistillation of the crude mixture obtained from the direct Rochowsynthesis, which consists in reacting methyl chloride over a catalystmass composed of silicon and a catalyst. This prior art says absolutelynothing about the possibility of employing heterogeneous catalysis overalumina to aid a specific redistribution reaction, as implemented in theprocess according to the invention, in which two pure organosilanes,separated beforehand from their preparation medium and consisting of achlorinated organohydrosilane and an organo-substituted and optionallychlorinated silane, are brought into contact.

The merit of the Applicant was not limited to this demonstration of thepossibility of employing heterogeneous catalysis over alumina in orderto promote a specific redistribution reaction between two organosilanestaken in the pure state. In fact, entirely surprisingly, the Applicantalso found that the alumina must meet well-defined characteristics inorder to perform correctly its role of heterogeneous catalyst in aspecific redistribution reaction.

The present invention therefore provides a process for obtainingorganosilanes, comprising a redistribution reaction between achlorinated organohydrosilane of formula (1) (R)_(a)(H)_(b)SiCl_(4-a-b)and an organo-substituted and optionally chlorinated silane of formula(2) (R′)_(c)SiCl_(4-c), in which formulae a 1 or 2, b=1 or 2, a+b≦3,c=1, 2, 3 or 4, the symbols R and R′ are identical or different and eachrepresent a linear or branched C₁-C₆ alkyl radical or a C₆-C₁₂ arylradical, the said redistribution reaction proceeding in the presence ofan effective amount of a catalyst based on a metal derivative, and thesaid process being characterized in that the catalyst remains in thesolid state in the presence of the reacting silanes (1) and (2) andconsists of an alumina which has an alkali metal M or alkaline earthmetal M′ content, expressed in ppm of oxide M₂O or M′O relative to theweight of catalyst (alumina comprising in particular M or M′), of lessthan or equal to 500 ppm.

In accordance with one preferred embodiment of the invention, thealumina used has an alkali metal or alkaline earth metal content of lessthan or equal to 300 ppm, preferably less than or equal to 100 ppm.

Owing to the major processes for their manufacture, the aluminasemployed in the present process usually contain sodium, the content ofwhich will therefore be expressed in ppm of Na₂O relative to the weightof alumina.

The Applicant notes that the alumina employed in its process, having analkali metal or alkaline earth metal content within the abovementionedranges, may advantageously comprise in its structure, in addition, adoping entity consisting of at least one halogen atom (such as, forexample, a chlorine atom) and/or of at least one atom of a metalselected from the group of the elements of groups 5b and 6b and mixturesthereof (such as, for example, niobium, molybdenum and tungsten), withthe proviso that the doping entity content, when it is present, andexpressed in % by weight of halogen atom(s) and/or metal atom(s)relative to the weight of catalyst (alumina comprising the dopingentity), is less than or equal to 50%, preferably less than or equal to30% and, more preferably, is within the range from 0.1 to 20%.

For the definition of the elements of groups 5b and 6b, reference ismade to the Periodic Classification of the Elements as published in“Handbook of Chemistry and Physics”, 51st edition, 1970-1971, edited by“The Chemical Rubber Co.”.

According to an even more preferred embodiment of the invention, thealumina used not only has an alkali metal or alkaline earth metalcontent and, if appropriate, a doping entity content which are withinthe abovementioned ranges but also possesses:

(i) a BET specific surface area greater than or equal to 50 m²/g, and

a total pore volume greater than or equal to 15 ml/100 g;

(ii) preferably

a BET specific surface area greater than or equal to 80 m²/g, and

a total pore volume ranging from 20 to 120 ml/100 g;

(iii) and still more preferably,

a BET specific surface area ranging from 100 to 600 m²/g, and

a total pore volume ranging from 25 to 80 ml/100 g.

The BET specific surface area is the specific surface area determined bynitrogen adsorption in accordance with the standard ASTM D 3663-78established on the basis of the Brunauer-Emmett-Teller method describedin “The Journal of the American Society” 60, 309 (1938).

The total pore volume (TPV) is measured as follows: the value of thegrain density (Dg) and of the absolute density (Da) is determined usingthe method of pyknometry, employing mercury in the case of the graindensity and helium in the case of the absolute density; the TPV is givenby the formula: ${TPV} = {\frac{1}{Dg} - \frac{1}{Da}}$

The alumina-based catalyst can be employed in various forms, such aspowder, beads, comminuted forms, extrudates or residues, in which caseshaping can optionally be carried out with the aid of a binder.

The alumina powder can be obtained by conventional methods, inparticular by rapid dehydration of a hydrated alumina or of an aluminiumhydroxide which is in the form, for example, of hydrargillite. Inparticular, the aluminas employed in the present process can be preparedby contacting a hydrated alumina in finely divided form with a stream ofhot gas at a temperature of between 400° C. and 1000° C., thenmaintaining contact between the hydrate and the gases for a periodranging from a fraction of a second to 10 seconds (a step known as“flashing”) and, finally, separating the partially dehydrated aluminaand the hot gases. Reference may be made, in particular, to the processdescribed in the American patent U.S. Pat. No. 2,915,365.

It is also possible to carry out autoclaving of the alumina agglomeratesobtained beforehand, in an aqueous medium and, optionally, in thepresence of acid, at a temperature of more than 100° C. and, preferably,of between 150° C. and 250° C. for a period which is preferably between1 and 20 hours and then to dry them and calcine them.

The temperature and the duration of the calcination are regulated suchthat the specific surface areas obtained are within the abovementionedranges.

The alumina can also be in the form of beads obtained from shaping by anoil-drop (or drop coagulation) technique. This type of beads can beprepared, for example, by a technique in accordance with the teaching ofthe patents EP-A-0 015 801 or EP-A-0 097 539. The porosity can becontrolled, in particular, in accordance with the technique described inthe patent EP-A-0 097 539, by drop coagulation of a suspension or of anaqueous dispersion of alumina or of a solution of basic aluminium saltwhich is present in the form of an emulsion consisting of an organicphase, an aqueous phase and a surfactant or emulsifier. The said organicphase can, in particular, be a hydrocarbon.

The alumina may also be present in one of its comminuted forms. Thesecomminuted forms can be obtained from the comminution of any type ofsubstance based on alumina, such as, for example, beads obtained by anytype of technique (oil drop, film coater or rotating drum) orextrudates. The porosity of these comminuted forms is controlled by thechoice of substance based on alumina which is comminuted in order toobtain them.

The alumina may also be in the form of extrudates. The latter can beobtained by grinding followed by extrusion of a substance based onalumina, the said substance possibly resulting from the rapiddehydration of hydrargillite or from the precipitation of an aluminagel. The porosity of these extrudates can be controlled by the choice ofalumina employed and by the conditions under which this alumina isprepared or by the conditions under which this alumina is ground beforeextrusion. The alumina can also be mixed with pore-forming agents duringgrinding. By way of example, the extrudates can be prepared by thetechnique described in the patent U.S. Pat. No. 3,856,708.

In the case where the alumina is doped with one or more elements, thesaid doping can be carried out by any method known to the person skilledin the art. It can be carried out, for example, by impregnating thealumina-based support with one or more precursors of these elements orby mixing the precursor or precursors with the alumina during theshaping of this substance.

In the case of doping, for example, by impregnation, this is done in aknown manner by contacting the support with a solution, a salt and/or agel comprising at least one element in oxide form, in salt form or inthe form of one of their precursors. The support can subsequently besubjected to an operation of drying and, optionally, of calcining; forexample, the catalyst can be calcined at a temperature of between 150and 1000° C., preferably between 200 and 900° C.

In the case of specific doping with a halogen, the precursor selectedwill be a halogen-containing mineral acid and, preferably, an organichalogen compound.

The catalyst is used in proportions by weight which range in generalfrom 0.1 to 10% and, preferably, from 0.5 to 5% relative to the totalweight of the silanes of formulae (1) and (2) introduced at thebeginning. Proportions by weight to which special preference is givenare those ranging from 0.8 to 2% relative to the same reference.

The temperature at which the redistribution reaction is implemented isgenerally greater than or equal to 130° C. The contact time between thesilanes of formulae (1) and (2) and the alumina is not critical and mayvary within wide ranges depending, in particular, on the apparatus, thestoichiometry of the reaction, and the chosen temperature.

Preferred temperature and contact time conditions are as follows:temperatures ranging from 140° C. to 260° C. and contact times rangingfrom 15 minutes to 8 hours. More preferred conditions are as follows:temperatures ranging from 150° C. to 240° C. and contact times rangingfrom 30 minutes to 5 hours.

From a practical standpoint, the process is conducted in a standard,closed reactor, which enables the liquid and/or gases to be contactedwith a heterogeneous catalyst while operating under autogenous pressure.The process can be implemented batchwise or continuously: in the firstvariant, there is no constraint regarding the employment of thereactants and of the catalyst, which can without disadvantage beprovided, in particular, in suspension in a liquid phase; in the othervariant, the redistribution reaction can advantageously be conductedcontinuously in a reactor, in particular a tube reactor, comprising thesolid catalyst arranged, for example, in a fixed or agitated bed.

Pressure is not a critical parameter of the process according to theinvention. It is therefore possible to operate under pressures rangingfrom 2 to 50×10⁵ Pa.

At the end of the heating period, when the redistribution reaction hastaken place (monitoring, for example, of the level of target chlorinatedorganohydrosilane from redistribution), the reaction medium is cooled toa temperature less than 40° C., preferably of between 10 and 30° C., andis then returned to atmospheric pressure, carrying out a degassingoperation if required. This produces a liquid phase, which is separatedfrom the solid catalyst phase comprising the chlorinatedorganohydrosilane from redistribution, which can be recovered inconventional manner, for example, by distillation under atmosphericpressure.

As far as the two types of silane which are introduced for reaction isconcerned, namely the chlorinated organohydrosilane of formula (1) andthe organo-substituted and optionally chlorinated silane of formula (2),it will be noted that the symbols R and R′ can be selected, for example,from the radicals methyl, ethyl, propyl, isopropyl, butyl, hexyl,phenyl, naphthyl and biphenylyl.

Preferably, the symbols R and R′ are identical or different and eachrepresent a linear or branched C₁-C₃ alkyl radical or a phenyl radical.

In any case, the symbols R and R′ which are especially preferred areidentical or different and each represent a methyl or a phenyl.

The process according to the present invention also applies to theimplementation of a redistribution reaction between the chlorinatedorganohydrosilane (1) of formula RHSiCl₂ and the organo-substituted andchlorinated silane (2) of formula R′₃SiCl (in this case a=1, b=1 andc=3), in which formulae the symbols R and R′ have the general meaningsgiven above, in the presentation of the invention, with respect toformulae (1) and (2).

The process according to the present invention applies particularly tothe implementation of a redistribution reaction between the chlorinatedorganohydrosilane (1) of formula RHSICl₂ and the organo-substituted andchlorinated silane (2) of formula R′₃SiCl (in this case a=1, b=1 andc=3) in which formulae the symbols R and R′ are identical or differentand each represent a linear or branched C₁-C₃ alkyl radical or a phenylradical.

The process according to the present invention applies especially to theimplementation of a redistribution reaction between the chlorinatedorganohydrosilane (1) of formula RHSiCl₂ and the organo-substituted andchlorinated silane (2) of formula R′₃SiCl, in which formulae the symbolsR and R′ are identical or different and each represent a methyl radical(abbreviated Me) or phenyl.

In general terms, in carrying out the process according to the presentinvention, the reactant of chlorinated organohydrosilane type of formula(1) can be present in the medium of the redistribution reaction in aproportion of at least 10 mol % relative to the mixture of chlorinatedorganohydrosilane of formula (1)+organo-substituted and optionallychlorinated silane of formula (2).

Preferably, the molar ratio$\frac{\text{chlorinated~~organohydrosilane~~of~~formula~~(1)}}{\text{organo-substituted~~silane~~of~~formula~~(2)}}$

is between 0.1 and 2. More preferably still, this molar ratio is between0.3 and 0.7.

In the context of the redistribution reaction to whose implementationthe process according to the invention applies especially, involving,for example, MeHSiCl₂ and Me₃SiCl as starting silanes (1) and (2), achlorinated organohydrosilane is finally recovered which is produced byredistribution and consists of Me₂HSiCl and the compound Me₂SiCl₂.

The examples which follow will make it possible to understand better allof the embodiments and advantages (processability) of the processaccording to the invention by emphasizing, by means of comparativetests, the small amount of Me₂HSiCl formed (percentage by mass less than5%) when the alumina employed does not correspond to the featuresaccording to the invention.

EXAMPLES 1 TO 3 AND COMPARATIVE TESTS A TO C

The process is carried out batchwise in a 75 ml cylindrical reactor madeof Hastelloy (a known material based on nickel) which is arrangedvertically, is agitated by shaking and is equipped with heating meansand with ports appropriate for the entry of a gas flow and for theintroduction of the reactants and catalyst.

The interior of the reactor is first of all subjected to a stream ofargon for 5 minutes and then charged in succession with

0.5 g of catalyst, in the form of a powder whose characteristics areindicated in Table I below, and

48 g of a mixture formed of Me₃SiCl (313 g) and MeHSiCl₂ (16.7 g) so asto give a molar ratio of MeHSICl₂/Me₃SiCl of 0.5.

TABLE I Specific Total pore Na₂O Doping entity surface area volumeAlumina ppm Type % m²/g ml/100 g 1 5 — — 408 39 2 30 Cl 1 210 60 3 30 —— 210 60 4 730 — — 244 50 5 1200 — — 171 55 6 4000 — — 210 59

The reactor is closed, agitation is commenced, and the reactor is heatedat 160° C. for 4 hours.

At the end of this period, the reactor is cooled by quenching in anice/water mixture for 5 minutes so as to bring its contents to atemperature of 20° C. It is then opened and the final reaction mixtureis recovered, the liquid phase of the said mixture being analysed by gaschromatography using a Varian instrument equipped with a catharometerdetector. The results obtained are reported in Table 2 below.

TABLE II Percentages by mass Examples Catalyst Me3 MeH Me2 Me₂H Ex. 1Alumina 1 48.5 5.0 33.3 8.2 Ex. 2 Alumina 2 48.0 3.2 33.0 7.9 Ex. 3Alumina 3 53.9 4.9 30.9 5.9 Compar. Ex. A Alumina 4 59.2 14.4 17.0 2.5Compar. Ex. B Alumina 5 65.1 27.4 3.6 0.3 Compar. Ex. C Alumina 6 64.026.7 6.1 0.1

Me3═Me₃SiCl; MeH═MeHSiCl₂; Me2═Me₂SiCl₂; Me2H═Me₂HSiCl

EXAMPLE 4

Example 2 is repeated but this time heating the reactor at 220° C. for 4hours. The results obtained are compiled in Table III below.

TABLE III Percentages by mass Example Catalyst Me3 MeH Me2 Me₂H 4Alumina 2 40.4 2.5 40.1 13.5

What is claimed is:
 1. Process for obtaining organosilanes, comprising aredistribution reaction between a chlorinated organohydrosilane offormula (1) (R)_(a)(H)_(b)SiCl_(4-a-b) and an organo-substituted andoptionally chlorinated silane of formula (2) (R′)_(c)SiCl_(4-c), inwhich formulae a=1 or 2, b=1 or 2, a+b≦3, c=1, 2, 3 or 4, the symbols Rand R′ are identical or different and each represent a linear orbranched C₁-C₆ alkyl radical or a C₆-C₁₂ aryl radical, saidredistribution reaction proceeding in the presence of an effectiveamount of a catalyst based on a metal derivative, wherein said thecatalyst remains in the solid state in the presence of the reactingsilanes (1) and (2) and comprises an alumina which has an alkali metal Mor alkaline earth metal M′ content, expressed in ppm of oxide M₂O or M′Orelative to the weight of catalyst (alumina comprising in particular Mor M′), of less than or equal to 500 ppm.
 2. Process according to claim1, wherein the alumina used has an alkali metal or alkaline earth metalcontent of less than or equal to 300 ppm.
 3. Process according to claim2, wherein the alkali metal or alkaline earth metal content is less thanor equal to 100 ppm.
 4. Process according to claim 1, wherein thealumina used comprises in its structure, in addition, a doping entitycomprising at least one halogen atom and/or of at least one atom of ametal selected from the group of the elements of groups 5b and 6b andmixtures thereof, with the proviso that the doping entity content,expressed in % by weight of halogen atom(s) and/or metal atom(s)relative to the weight of catalyst (alumina comprising the dopingentity), is less than or equal to 50%.
 5. Process according to claim 4,wherein the doping entity content is less than or equal to 30%. 6.Process according to claim 5, wherein the doping entity content iswithin the range from 0.1 to 20%.
 7. Process according to claim 1,wherein the alumina possesses: a BET specific surface area greater thanor equal to 50 m²/g, and a total pore volume greater than or equal to 15ml/100 g.
 8. Process according to claim 7, wherein the aluminapossesses: a BET specific surface area greater than or equal to 80 m²/g,and a total pore volume ranging from 20 to 120 ml/100 g.
 9. Processaccording to claim 8, wherein the alumina possesses: a BET specificsurface area ranging from 100 to 600 m²/g , and a total pore volumeranging from 25 to 80 ml/100 g.
 10. Process according to claim 1,wherein, the catalyst is used in proportions by weight which range from0.1 to 10% relative to the total weight of the silanes of formulae (1)and (2) introduced at the beginning.
 11. Process according to claim 10,wherein the proportions by weight range from 0.5 to 5%.
 12. Processaccording to claim 1, wherein the temperature at which theredistribution reaction is implemented is greater than or equal to 130°C.
 13. Process according to claim 12, wherein the temperature is withinthe range from 140 to 260° C.
 14. Process according to claim 1, wherein,in relation to the two types of silane which are introduced forreaction, namely the chlorinated organohydrosilane of formula (1) andthe organo-substituted and optionally chlorinated silane of formula (2),the symbols R and R′ are identical or different and each represent alinear or branched C₁-C₃ alkyl radical or a phenyl radical.
 15. Processaccording to claim 1, wherein a redistribution reaction is carried outbetween the chlorinated organohydrosilane (1) of formula RHSiCl₂ and theorgano-substituted and chlorinated silane (2) of formula R′₃SiCl (inthis case a=1, b=1 and c=3).
 16. Process according to claim 1, whereinthe molar ratio$\frac{\text{chlorinated~~organohydrosilane~~of~~formula~~(1)}}{\text{organo-substituted~~silane~~of~~formula~~(2)}}$

is between 0.1 and 2.